1. Field of the Invention
This invention relates a method for detecting focus of an image, namely for detecting whether or not a picture image recorded on a recording medium is out of focus and more particularly to a method for detecting whether or not the picture image recorded on a photographic color film is out of focus.
2. Description of the Prior Art
In the printing of pictures taken by amateur photographers, it is necessary to eliminate those photographs which are so improperly exposed as to make compensatory correction in the printing process impossible and those photographs which are out of focus. This is particularly true in the case of the printing of color photographs both because of the high price of the photographic materials used and because of the need to conserve natural resources.
There are already in use a number of general methods for detecting whether or not a picture image is out of focus. One of these methods subjects the picture image recorded on the photographic film to Fourier transformation to obtain information regarding the spatial frequency region of the picture image and determines whether or not the picture image is out of focus on the basis of this information. Another determines whether or not the picture image is out of focus from the ratio between the maximum density gradient of the whole picture image, which is obtained from the density values of picture elements (meaning minute portions into which the image is divided beforehand), and the maximum density gradient for the part of the picture image remaining after elimination of the high spatial frequency region (see U.S. Pat. No. 4,379,632). Still another is the method employing a blur mask proposed in U.S. Pat. application Ser. No. 251,099 filed Apr. 6, 1981.
A given photographic image almost always includes objects that were at various distances from the camera at the time the photograph was taken. For this reason, photographs in which all objects in the picture image are in focus are exceedingly rare. Therefore, if the main object of the photograph is in focus, the picture image can be said to have good clarity even though the other objects included therein are out of focus. With the conventional methods such as those mentioned above, however, the judgment as to focus is made on the basis of information concerning the whole picture image and, therefore, even a photograph whose subject is in good focus will be judged to be out of focus if other objects in the photograph are out of focus.